There has been a lot of discussion lately about the merits, or lack thereof, of armor class (and by extension hit points) in the d20 game. Armor class as an abstraction was done for a very simple reason, it was fast. It didn’t take a lot to figure out whether an attack was successful or not or whether that attack did damage. I do not disagree that it was possibly an over simplification and it is certainly wildly inaccurate. To be blunt if accuracy is what you want play GURPS. The d20 construct was never intended to be an accurate representation of combat or of much of anything really. Interestingly though despite the undeniable fact that there are more accurate game systems available d20 in its many versions and forms is still a popular and viable game system while some others have fallen into what can best be described as a niche existence. Part of the reason for that popularity is that we all know it. D&D is ubiquitous in the gaming world.
Fortunately for those of us who like the d20 construct but would like something a little more accurate the wonderful folks at Paizo have provided an alternative to armor class. The Advanced Combat Guide provides variant rules for armor class and hit points. The purpose of this JiB on GM’ing article is to explore those rules a bit.
First off a bit of explanation
Though we all have used armor class over the years it is useful to have a common understanding and so, “Armor class is an expression of how hard a character is to harm and is a product of the character’s dexterity plus any defenses they may have. Namely armor and shields.” This is my working definition of armor class for our purposes. I’m not going to concern our discussion with magical defenses because all they do is alter the numbers. They do not alter the basic equation. Since we brought up the concept of an equation here it is:
Armor Class = 10 + Armor Modifier + Shield Modifier + Dexterity Modifier
Which means that a character with a 14 Dex (+3), and wearing chainmail (+5) and carrying a heavy shield (+2) is going to end up with:
20 = 10 + 5 + 3 + 2
Which means that a monster with an attack bonus of +7 will need to roll a 13 or better on the die to hit. Which brings us to the math portion of our program. The monster will be successful approximately 35% of the time.
.35 = 7/20
Every shift of +/- 1 on either the character’s armor class or the monsters attack is a 5% shift one way or the other. Admittedly not very accurate, but easy to work with.
So what about armor as damage reduction
If we are to use armor as damage reduction in d20 it will need to still work with all of the magic items and spells that affect armor class. Fortunately the folks at Paizo are smart because in chapter 5 (Variant Rules) of the Ultimate Combat Guide they have an answer.
First throw out the concept of armor class alltogether. Replace it with a Defense score. Semantics but sure why not.
“In this alternative system, a creature does not have an Armor Class (AC); it instead has a Defense score. Defense is similar to touch AC in the standard Pathfinder Roleplaying Game rules, but it also adds the shield bonus (including any enhancement bonus to the shield), and any enhancement bonus to armor.” UCG p.191
Defense = 10 + shield bonus + Dexterity modifier + other modifiers (including armor’s enhancement bonus but not armor bonus or natural armor bonus)
Which means that our example character would end up with a Defense of 15
15 = 10 + 3 + 2
This leaves our character 25% more likely to be “touched” by an attack than if we were using armor class. But we’re not done yet.
Because of his chainmail our hero receives dr/5 armor so he gets to ignore the first 5 points of most attacks. Note that some creatures and some attacks bypass dr.
So let’s try an example (Oh quit complaining you knew it was coming)
Our hero is fighting a goblin and the goblin attacks with a cleaver (Why the goblin has a cleaver is anyone’s guess but he does.) For our purposes we’ll treat his cleaver as a dagger.
The goblin rolls his attack and gets a 17. If we were using AC the attack would miss and we would be done. But using armor as dr the 17 touches our hero so the goblin rolls damage and gets a 4. The four is less than the dr of his armor so the attack has no effect. The net result is the same. In aggregate (and somewhat anecdotally) it seems that attacks are somewhat more likely to do damage than with armor class but that the overall amount of damage is actually reduced. Which leads into the next JiB on GM’ing article, “Wounds and Vigor vs Hit Points.”
Based on the tests that I have run the results come out about the same either way. It is a bit more work to do armor as dr but not onerously so. Whether it is a better representation than armor class is a question I leave for the reader to answer. Personally I like it and am implementing the variant rules for the Pathfinder games that I run in the future.
Next I will take a look at Wounds and Vigor vs Hit Points.
It speeds up combat to roll the attack and the damage in the same cast so for the goblin attack above I would cast a d20 and a d4 at the same time. I tend to color group dice so if I had multiple attacks each attack has a color of its own.
Filed under: Misc